The present invention relates to a tracking servo system for tracing the center of a track by a light spot and, more particularly, to a push-pull tracking servo system based on a differential diffraction method which traces a track by utilizing diffracted light obtained therefrom. The present invention is adapted for use in an optical code data memory of a recordable/reproducible adding type or erasable type optical disc system.
In the conventional optical information recording and reproducing apparatus, there is known one example as disclosed in U.S. Pat. No. 3,909,608, wherein a tracking signal for tracing a desired track is produced by differentially processing the outputs of two photodetectors which are disposed in parallel with the track and sense the diffracted light obtained from the track. However, since the photodetectors are disposed in the far field of the diffracted light, a disadvantage has been existent heretofore that, if a light beam is deflected or a disc is tilted, an offset is generated in the tracking signal to eventually fail in achieving a proper tracking operation.
In a tracking signal detector utilizing such diffracted light, correction of any offset generated in the tracking signal due to the tilt of the disc can be executed by one known method disclosed in Japanese patent Laid-open No. 59-19250 (U.S. patent application Ser. No. 515,520 filed on July 20, 1983, now U.S. Pat. No. 4,663,751), according to which mirror areas are provided in the track. This prior art comprises extracting a mirror-area signal included in the tracking signal and then applying such extracted signal to correct the offset derived from the tilt of the disc. In the above method, however, the object resides merely in correction of the offset caused by the tilt of the disc.